In the manufacture of semiconductor devices, many processes are carried out in a furnace commonly referred to as a diffusion furance, although such furnaces are not limited to diffusion operations. The furnace typically consists of an elongate round or rectangular tube having a length to diameter ratio in excess of 5 to minimize the effects of heat loss from the tube ends. An electrical heating element surrounds the tube and may be formed with independently-controlled zones to maintain a desired temperature profile in a chamber defined by the central portion of the tube. Such tubes often have a necked down end to accept one or more input tubes carrying gases and have their opposite ends open to a scavenger area for exhaust of the process gases.
In a typical process, a number of semiconductor wafers (e.g., thin slices of single-crystal silicon) are placed in a carrier, called a boat, either in a substantially vertical or substantially horizontal position, and the boat is then inserted into the central, chamber portion of the furnace. Process gases are introduced at one end, pass over the wafers, and exhaust out the other end. The desired reaction occurs when the gases are in contact with the wafers. In the case of deposition on the wafer surface, the gases are heated at the surface or in the immediate vicinity and react. The desired material is deposited while the other reaction products pass downstream over the other wafers to the exit. For example, to deposit a film of silicon nitride, separate gas streams of tetrachlorosilane and ammonia are introduced at one end of the tube and in passing over the wafers, the gases react to form silicon nitride and hydrogen chloride. The silicon nitride adheres to the wafer while the hydrogen chloride is exhausted. In the case of a diffusion process, one or more dopants are introduced in one or more gaseous streams for deposition on the wafers. The wafers are then held at an elevated temperature in a controlled atmosphere while the dopant diffuses into the wafer substrate.
In all deposition processes, a major concern is to achieve uniformity of the deposited material, both in thickness and composition, over the entire surface of each wafer, over all wafers in the furnace and over all successive batches of wafers. It can be appreciated that uniformity is a major problem in the processes described above because of variations in gas composition, temperature and gas velocity as the gases proceed from one end of the tube to the other. After the gases react on the first wafers, their compositions are locally altered so that succeeding wafers are treated with different gas compositions. In diffusion processes, semiconductor material is doped with a chemical element such as boron or phosphorous so as to produce certain desirable electrical porperties. The level of doping is commonly very small, in the range of a few parts per million. Since the process temperature is frequently near the softening point of the substrate, there is a tendency for the doping atoms to migrate out of the substrate into the gas stream, and vice versa. In a typical horizontal furnace, atoms which evaporate may be carried downstream and deposited on other wafers, causing variations in electrical properties. The foregoing problems can be somewhat alleviated by angling the wafer carrier, by canting the wafers in the carrier and/or by in other ways changing the flow geometry so that fresh, unreacted gas is constantly mixed into the stream.
In a second cause of nonuniformity, as the gases pass over the heated wafers and along the inside of the heated tube, they become progressively more heated and, therefore, their reaction rates will vary. Some compensation may be obtainable by adjusting the temperature profile of the furnace or by preheating the gases. Variations in gas velocity from one wafer to another is a third cause of uniformity, affecting the available chemical reaction time to each wafer.
For all of the foregoing reasons, the deposition process requires considerable technical skill to adjust it properly in the beginning, and excellent control of temperature, gas flow rate, gas composition and process time in order to achieve reproducable results.
The present invention provides apparatus and methods for accomplishing the deposition function with larger batch sizes of wafers or other units of semiconductor material than generally treated, yet with superior uniformity, without the necessity of excessive control of temperature, gas flow, gas composition and process time. The process may be characterized as a radial flow, or cross-flow, process in contrast to the normal methods described above which are axial flow processes.
In particular, apparatus and processes are provided for distributing gas to a plurality of at least three discrete space locations within a heated chamber for cross-flow of the gas so that it travels across the longitudinal axis of the chamber along a plurality of spaced paths for single pass traversal thereof. In a specific embodiment the gaseous reactants enter the furnace through a pair of distribution tubes that are closely adjacent to the upper surface of the tubular furnace wall. The distribution tubes are manifold tubes formed with small orifices which are aligned tube-to-tube so that gases from adjacent tubes converge as they flow from the tubes to thorourghly mix in a very small space.
The apparatus particularly enables the processing of a large number of wafers, each wafer having a substantially planar surface. The wafers are disposed so as to be vertically directed in a carrier of open construction with their planar surfaces preferably substantially vertically extending and substantially normal to the longitudinal axis of the reactor chamber which allows gases from the manifold tubes to flow essentially unimpeded parallel to the surface planes of the wafers. In the event that desposition is to occur on only one surface of each wafer, the wafers can be placed in back-to-back disposition. The carrier rests in the tubular furnace in such a manner as to form a plenum area under the wafers from which the spent or unused, cross-flowed gases are exhausted to one or both ends of the furnace. This arrangement allows each wafer to be treated with the same gas composition and flow conditions, while avoiding communication via the gas streams between the individual units, and thus significantly improves the uniformity of the process. In addition, many more wafers can be loaded in a batch since the gas does not pass successively over more than one wafer.
In a specific embodiment, the manifold tube is closed at one end. One or more imperforate conduit tubes are disposed within and extend along the manifold tube so that process gas can be delivered along a length of the heated chamber to a location ajdacent the closed end of the manifold tube for counterflow in the manifold tube from the closed end to the orifices. By such means, the process gas flows from one end to the other of a heated section and then exits the orifices on the return flow. The result is a counterflow heat exhange to provide substantially uniform gas temperature at the manifold orifices from one end of the tube to the other end.
In alternative embodiments, the present invention provides other input arrangements including the use of a manifold tube from each end of the heated chamber, an input tube doubling back on itself with orifices located on the return portion, use of a single manifold tube, or a series of small manifold tubes, and use of one or more imperforate conduit tubes in parallelism or axially concentric within the manifold tube for conveyance and mixing of different gases in the manifold tube.